1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring the coordinates of the edges of a tape. More specifically, the invention pertains to a method of measuring the coordinates of the tape edges and an apparatus for carrying out the method, the method comprising the steps of attaching a tape to a three-dimensional subject by adhesion or the like such that one edge of the tape is positioned along one edge of the subject, the tape having a reflectance different from that of the subject, automatically detecting the edge of the tape in a non-contact manner; and tracking the thus-detected edge, thereby measuring the coordinates of the edge of the tape in a non-contact manner.
2. Description of the Prior Art
During production of certain kinds of structure, machine part, die (press die, forecasting die) or model, it is well known that the three-dimensional coordinates of an edge of such a subject where surfaces thereof meet are employed in the form of data representative of the configuration of the subject as an inspection or production reference. For example, in a case where a model of a vehicle body or a die thereof is produced, the configuration of its surface is generally a freely curved surface. Therefore, it is common practice to employ the three-dimensional coordinates of an edge as a production reference or an inspection reference for the surface. In particular, in the case of a model or a die of the external appearance of the vehicle body, the configurations of each edge, after production, serves as a characteristic curve which greatly influences the external appearance of the vehicle body. Accordingly, the coordinates of the edges are measured as particularly important data, and thus various inspections need to be performed based on the data.
The general surfaces and freely curved surfaces of the three-dimensional subject are automatically measured by means of a three-dimensional-coordinate measurement machine. However, the coordinates of edges are conventionally measured in a contact manner by adjusting a measurement probe to the apex of each of the edges with the eye. In a process in which the surface is automatically worked and measured, a large number of steps are required to measure the edges. In addition, since this measurement is manually performed, the accuracy of measurement varies due to large errors involved in the measurement. Moreover, it is likely that a scratch may be formed on a surface to be measured.
In another known method, the position or configuration of a three-dimensional subject is measured in a non-contact manner. In this method, a beam of light is projected onto a surface of the subject, and the locus of the beam which is bent at the edge is observed through a TV camera (an integration type image-pickup tube) at a certain angle to perform triangulation, thereby obtaining data representing the edge.
In this known method, however, it becomes necessary to prevent the image from being dimmed due to variations in the distance from the subject. More specifically, in a case where a dim image is projected onto several pixels of the TV camera, several kinds of image processing such as thinning are needed in order to accurately obtain the position of the image. This may lead to the problem that the speed of processing is reduced due to the time spent for such image processing. Also, where surfaces other than completely diffusing surfaces are measured, it is impossible to obtain an accurately measured value due to various influences such as mirror reflection.